As is well known, the compressor case of a gas turbine engine powering aircraft is subjected to severe pressure and temperature loadings throughout the engine operating envelope and care must be taken to assure that the components remain concentric while maintaining relatively close running clearances so as to avoid inadvertent rubs. Inasmuch as the engine case is thin relative to the rotor and stator components in the compressor section, it responds more rapidly to temperature changes than do other components. This is particularly true during periods of transient engine performance. Typical of these transients are throttle chops, throttle bursts, and the like. Obviously it is customary to provide sufficient clearances during these transients to assure that the rotating parts do not interfere with the stationary parts.
The problem becomes even more aggravated when the engine case is fabricated in two halves (split case) which is necessitated for certain maintenance and construction reasons. Typically, the halves are joined at flanges by a series of bolts and the flanges compared to the remaining portion of the circumference of the case is relatively thick and hence does not respond to thermal and pressure changes as quickly as the thinner portion of the case. The consequence of this type of construction is that the case has a tendency to grow eccentrically or out of round.
In certain instances, in order to attain adequate roundness and concentricity and to achieve desired clearance between the rotating and non-rotating parts, it was necessary to utilize a full hoop case for the highest stages of a multiple stage compressor. Since the stator components, i.e., stator vanes and outer air seals, are segmented the problem was to assure that the compressor maintained its surge margin notwithstanding the fact that the outer case would undergo large deflection at acceleration and deceleration modes of operation. The cavity that exists between the outer case and the inner case formed by the segmented stator components, being subjected to pressures occasioned by the flow of engine air through the various leakage paths, presented a unique problem. In the event of a surge, which is a non-designed condition, the pressure in the gas path would be reduced significantly. Because the air in the cavity is captured and cannot be immediately relieved, it would create an enormous pressure difference across the stator components, cause them to distort, with a consequential rubbing of the compressor blades, and a possible breakage.
In order to withstand this pressure loading and yet achieve the roundness and clearance control of the stationary and rotating components, it was necessary to incorporate a mechanism that would tie the outer case to the segmented stator components. While it became important to assure that this rubbing did not occur, particularly where severe rubbing could permanently damage the blades and/or rotor/stator during surge, the mechanism that is utilized must be capable of withstanding this enormous load, yet be insensitive to fatigue.
Moreover, in order to achieve roundness and maintain close tolerance between the tips of the blades and outer air seal, it is abundantly important that the components subjected to high thermal and load differentials do not allow the outer and inner cases to grow eccentrically. Conventionally, the inner segmented case at the last stage of the compressor is supported to the outer case by an annular, wish-bone shaped bulkhead. Applicable flanges formed on both cases are suitably bolted to cooperating flanges on the wish-bone element. This construction is shown in FIG. 1, which exemplifies a prior art construction showing a typical axially split outer case carrying flange 6 bolted to wish-bone support member 7. Each of the circumferential segments 8 (only one shown) carries flange 9 which is likewise bolted to the wish-bone support member 7. The wish-bone configuration provides relative flexibility between the outer case 5 which is relatively cool and the inner case 8 which is relatively hot since the outer case sees cooler fan air and the inner case sees engine gas path air. It is apparent from the foregoing that this construction allows for any distortion to be taken up by the flexibility provided by the wish-bone configuration.
In the circumferential direction, the segments 8 are bolted to the wish-bone element 7 to form a rigid, almost unitary assembly and the growth, due to loads and thermals, is taken up by the characteristics inherent in the wish-bone shape of element 7. In heretofore designs, each segment included at least three circumferentially equispaced bolts.
By those considered experts in this field of technology, this type of bulkhead construction, sometimes referred to as the backbone, was the only suitable way to compensate for the high loads and temperature ranges at this location of the engine and yet meet acceptable low cycle fatigue (LCF) specifications.
We have found that we can achieve acceptable LCF life, obtain a less complicated and lower weight backbone construction and eliminate the costly wish-bone shaped element. To this end, a relatively straight shaped annular backbone supports the inner segmented case to the outer case. This arrangement enhances the control of the clearances between the tips of the blades and outer air seal.